The use of implantable cardioverter defibrillator (ICD) devices that deliver an electrical counter shock from a charge stored in a high voltage capacitor system so as to treat cardiac arrhythmia is well known. The capacitor systems in all present ICD devices which have been approved for use in humans utilize capacitors that are of the aluminum electrolytic type. While aluminum electrolytic capacitors provide high energy density per unit volume, they tend to degrade electrochemically over time and need to be reformed frequently to maintain optimum capacitor-discharge. This need for frequent reforming results in the need for larger power supplies for the ICD, which in turn increase the size of the ICD. Accordingly, it would be desirable to provide a capacitor maintenance system for aluminum electrolytic capacitors which decreases the power demands of frequent reforming.
ICDs use capacitor-discharge systems that generate high energy cardioversion/defibrillation countershocks by using a low voltage battery to charge a capacitor system over a relatively long time period (i.e. seconds) with the required energy for the defibrillation countershock. Once charged, the capacitor system is then discharged for relatively short, truncated time period (i.e. milliseconds) at a relatively high discharge voltage to create the defibrillation countershock that is delivered through implantable electrode leads which discharge current into the heart muscle of a human patient.
Presently, all capacitor-discharge ICDs are designed such that the capacitor system can store a maximum electrical charge energy of at least about 25 joules. This requirement imposes a need to maintain the capacitor system at optimum performance levels. Accordingly, aluminum electrolytic capacitors must be reformed and maintained at peak capacity. A typical reforming regimen is to charge the capacitor up to its full rated voltage and then allow the accumulated voltage to trickle off. This process electrochemically reforms the oxide in the capacitor anode so as to enable a more efficient capacitive discharge. When the capacitor system has gone a long time without reforming, the charge time to attain a maximum voltage on a charge can increase significantly. Typically, for example, the charge time could increase from five seconds to ten seconds in a period of one or two years. A worst case scenario is when the charge time could be as long as thirty seconds or more when the battery system used to charge the capacitor system is near its end of life. This increase in the charge time could be detrimental to a human patient who needs a full charge defibrillation shock immediately.
Early ICD devices contained robust battery systems with large energy reserves. Therefore, these devices had excessive battery energy which was not affected by the need to reform the capacitor system. Further, these devices had a short service life which made reforming a less critical energy concern because the devices were discarded long before the battery energy was exhausted. In sharp contrast, current ICD devices use compact batteries with a limited energy budget that are designed to last for a relatively long time. Both space and volume requirements favor small batteries in ICDs. The state of the art in battery technology, however, is such that compact batteries do not hold a large energy reserve. Capacitor reforming using these small batteries can consume a significant portion of the scant energy reserve. Presently, the automatic reforming features of all ICD's are based on a set time period. This automatic reforming strictly uses time intervals between scheduled maintenance cycles to determine when to reform the capacitor system.